1. Technical Field
The present invention relates to data transmission, and in particular to data transmission over an electricity distribution network.
2. Background Information
Electricity distribution networks are extensively used to transmit electricity from electricity generating stations, such as power stations, to domestic and industrial consumers. It has been realised that the electricity distribution network provides an infrastructure connecting thousands of consumers which can be used to transmit more than just electricity. For example, the distribution network can be used for the transmission of data.
Existing systems in which the electricity distribution network is used to transmit data are called ‘Power Line Communications’ (PLC) and ‘Broadband over Power Lines’ (BPL). In these systems a high frequency data signal is added, or inserted, onto an electrical power cable using an inductive coil or capacitor arranged around or near the cable. A corresponding inductive coil or capacitor is then used to remove, or extract, the data signal at the other end. These systems allow a data signal to be transmitted between two parties over an existing power cable without the need for a separate data communication infrastructure. Furthermore, the transmission of data does not interfere with the primary use of the cable, that is to transmit electricity.
There are however a number of problems associated with implementing PLC on existing electricity distribution networks, particularly where the infrastructure is old or where the parts are inaccessible.
In many parts of a distribution network (such as high voltage overhead power cables) the electrical cables are completely exposed or are coated with a thin protective layer or sheath. In these situations, access to the cables is largely unhindered and as a result data can be injected onto the cable easily using a small inductive coil. However, there are many parts of a network in which the cables are shielded for insulation and/or protection purposes such as shielded cables used in underground applications. Shielded cables typically comprise a core which carries the power and which is surrounded by a metallic sheath and a series of mild steel windings or tape (referred to as armour) running around and along the length of the cable. In situations where cables are shielded, large inductive coils or capacitors are needed to inject the data signal through the cable shield and to the conductive cores of the cable. It is of course possible to use large inductive coils to inject a data signal through the shield but installation and mounting of these coils on existing cables has made their use commercially unviable. Furthermore, the quality of the data signal which can be injected onto or extracted from a shielded cable can be diminished due to the cable shield and armour configuration.
The problems of using armoured cables in PLC applications is a limiting factor wherever access to the cable core is required to inject or extract a data signal. For example, PLC networks often require repeaters to ‘boost’ the signal strength where the signal is required to travel over extended distances. Again, armoured cables mean that access to cable cores to inject the signal requires a substantial increase in the cost of PLC repeaters.
The problems discussed above also arise when an electrical cable is connected to apparatus such as transformers, termination boxes and the like. In these situations the cables are shielded (using mild steel armour) for safety purposes. In a conventional connection of a shielded electrical cable to a termination box, the shielding of the cable is mechanically coupled to the termination box by a mechanical clamp. The clamp grips the steel armour to mechanically support the cable against the termination box housing and prevents the cable pulling away from the termination, box, also electrically bonding the armours to the potential of the metallic shield. The cores of the cable, which are surrounded by the armour and insulation, feed directly into the termination box thereby providing the required electrical path.
Because termination boxes are often sealed and inaccessible the only way to implement PLC in these situations is to install an inductive coil around the shielded cable before the cable enters the termination box.
In general, the use of shielded cables with inaccessible cable boxes within an electricity distribution network restricts access to the cable cores and makes the implementation of PLC to these parts of the network difficult and expensive. One option to overcome the problems of implementing PLC with shielded cables is to replace each, of the transformers or termination boxes with an equivalent which provides access to the cable core. However, it will be appreciated that the cost of replacing transformers and termination boxes in an electricity distribution network is both disruptive to the electricity supply and extremely expensive.
There is therefore a need for apparatus which allows PLC to be implemented on an existing distribution network in which shielded cables are used without the need for substantial modifications to the equipment. Specifically, there is a need for apparatus which removes the requirement for large and expensive network modifications and which does not compromise the electrical safety or the mechanical integrity of the connection.